A fuel cell has high power generation efficiency and is an environment-friendly power generation system because of being low in discharge. In accordance with the recent increase in attention on global environment protection and departure from dependence on fossil fuel, the fuel cell attracts lots of attention. The fuel cell is expected to be mounted on, for example, small-scale decentralized power generation facilities; a power generation system serving as a driving source of a movable object such as an automobile or a ship; or a mobile phone or a mobile personal computer, instead of a secondary cell such as a lithium-ion cell.
A polymer electrolyte fuel cell has a pair of electrodes on both sides of a proton-conductive solid polymer electrolyte membrane and obtains electomotive force by supplying pure hydrogen or modified hydrogen gas serving as fuel gas to one of the electrodes (fuel electrode) and supplying oxygen gas or air serving as an oxidant to the other electrode (air electrode). Furthermore, water electrolysis produces hydrogen and oxygen by an inverse reaction of the fuel cell reaction by electrolyzing water with the solid polymer electrolyte membrane.
However, in an actual fuel cell or water electrolysis, in addition to these main reactions, side reactions occur. A typical side reaction is generation of hydrogen peroxide (H2O2). Radical species derived from the hydrogen peroxide is a cause of deteriorating the solid polymer electrolyte film.
Conventionally, as the solid polymer electrolyte membrane, perfluorosulfonic acid membrane, which is commercially available under a trade name of Nafion (registered trademark, manufactured by DuPont), Asiplex (registered trademark, manufactured by Asahi Chemical Industries Co., Ltd.), or Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), has been used because of its excellent chemical stability.
However, the perfluorosulfonic acid membrane, such as Nafion, is difficult to be produced and therefore has a problem that it is very expensive. This is a large obstacle preventing the membrane from being widely applied to consumer purposes, such as fuel cell automobiles and household fuel cell power generation systems. In addition, since many fluorine atoms are included in the molecule, it has a problem that disposal treatment after use is also a heavy burden on the environment.
In addition, in the fuel cell, the membrane resistance is reduced by being operated at higher temperature and being reduced in thickness of the proton-conducting membrane between the electrodes, and the output power generation can be thereby increased. However, the perfluorosulfonic acid membrane has a heat deformation temperature of about 80 to 100° C. and, thereby, is extremely poor in creep resistance at high temperature. Therefore, the temperature for power generation must be maintained at 80° C. or less when the membrane is used in a fuel cell. Thus, there is a problem of limitation in output power generation. In addition, the stability of membrane thickness in long-term use is poor. Therefore, in order to avoid short circuit (short) between electrodes, a certain thickness (50 μm or more) is necessary, and it is believed that it is difficult to thin down the thickness.
In order to solve these problems of the perfluorosulfonic acid membrane, many studies on solid polymer electrolyte membrane, not containing fluorine atoms, being more inexpensive, and having a thermally stable main chain skeleton, such as one that is also used in engineer plastics, have been currently conducted. Polymers in which a polyarylene-based, polyetheretherketone-based, polyethersulfone-based, polyphenylene sulfide-based, polyimide-based, or polybenzazole-based main chain aromatic ring is sulfonated have been proposed (Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490-2492 (1993): Non-patent document 1, Polymer Preprints, Japan, Vol. 43, No. 3, pp. 735-736 (1994): Non-patent document 2, Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993): Non-patent document 3).
However, these polymers whose main chain aromatic rings are sulfonated have high water absorbability and are poor in hot water resistance. Therefore, the degree of introduction of a hydrophilic group, such as a sulfonic acid group, is limited. Furthermore, the polymers are materials that are poor in resistance to a Fenton reagent (resistance to hydroxy radicals) serving as a scale of power generation durability. In addition, when an electrolyte membrane of such a polymer is exposed to a high temperature of 100° C. or more for a long period of time, the sulfonic acid is eliminated to cause a reduction in proton conductivity or to bring about a cross-linking reaction with another aromatic ring to which a sulfonic acid group is not introduced, resulting in a problem of embrittlement. If the embrittlement of the membrane progresses, rupture (pinhole) occurs in the membrane during long-term power generation, resulting in a high possibility of impossible power generation.    Non-patent document 1: Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490-2492 (1993)    Non-patent document 2: Polymer Preprints, Japan, Vol. 43, No. 3, pp. 735-736 (1994)    Non-Patent Document 3: Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)